If you have a Nexus One, you may have received a notification that a over-the-air update for Android 2.2 FRF85 (or Froyo) is available for download. However if you haven't you can download it from Google's servers to update from EPE54B to FRF85.

Alternatively, here is a direct link to download a FRF85 build that'll supposedly work for any Android build. Engadget has confirmed that it worked for them on a T-Mobile Nexus One with 2.1 / ERE27. Download it from Google here. Also go here to see the official report from Google about the update.